Inboard marine engines require exhaust systems to carry the hot exhaust to the exterior of the boat. Engines, especially diesel engines, generate a considerable amount of heat which is dissipated through the exhaust. The heat becomes a problem because the exhaust system is contained within the hull of the boat. Exhaust elbows are mounted to the exhaust manifold of the engine wherein seawater is added to the exhaust. The water channel usually externally jackets the circular exhaust pipe and mixes with the exhaust gas with a cooling effect. The more the water is atomized and vaporized the cooler the exhaust becomes. The cooling effect of vaporized water is best demonstrated in evaporative coolers which have existed since 1937. The exhaust elbow cooling system is most effective at high engine speeds wherein large volumes of seawater flow through the exhaust. Cooling the exhaust lines at low engine rpm speeds (1600–2000 rpm) is a problem because of the low volume of water. With low seawater flow through the exhaust elbow the seawater only flows through in the lower part of the exhaust elbow which produces very little atomization and vaporization of the water which is necessary for the vaporization cooling effect. The exhaust heat at low engine speeds has caused early deterioration of the silicone-acrylic exhaust tubes which are used more commonly with diesel engines. This is particularly evident in the wet exhaust system of the Nordhavn 35 coastal pilot which is designed to maximize working space of the engine room. The 5″ rubber exhaust hose is routed to the water lift box mounted in the Lazarett. The exhaust hose has a history of damage at low engine rpm (1600–2000 rpm), the reduced water flow through the exhaust elbow does not provide a water spray pattern for sufficient heat exchange of the exhaust gases.
In order to overcome the overheating problems water has been injected at the upper part of the water jacket. In U.S. Pat. No. 6,293,121 atomizer nozzles are used to maximize the water surface. Both methods have limited success and longevity, the heat of the exhaust and corrosiveness of the seawater combine to close the water orifices.